Useful Application of Function 'Reduce Number of Generated Combinations'
Tips & Tricks
This post describes two practical examples based on the Eurocodes where the reduction of combinations is reasonable. There is a large number of various National Annexes as well as several material standards (EC 2 to EC 9) that are not in compliance with the rules for structural design (EC 0).
Example 1: Selecting leading variable actions
According to DIN EN 1990, the ultimate limit state designs in case of fire have to be performed in the exceptional design situation. The national application document also leaves open the question whether the frequent value (psi_1.1 x Q_k, 1) or the quasi-permanent value (psi_2.1 x Q_k, 1) should be used. The National Annex to DIN EN 1991-1-2 in the Nationally Defined Parameter (NDP) for 4.3.1 (2) provides information here. It applies here that for structural components whose leading action is the wind, the frequent value (psi_1,1 x Q_k, 1) has to be applied. Otherwise, you can use the quasi-permanent value (psi_2,1 x Q_k, 1).
In RFEM/RSTAB, it is possible to select the combination rules between both values. For the fire case, the combination rule "Exceptional - psi 1,1 - Eq. 6.11c "and only the wind can be defined as the leading action in the" Select Leading Variable Actions ... "item. Thus, only the wind with the frequent variable and the remaining variable actions with the quasi-permanent variable are considered in the combinations.
Example 2: Reducing number of load cases
For the serviceability limit state design, the British National Annex to BS EN 1993-1-1 (Steel Structures) requires the characteristic combination without components of the permanent loads (see BS EN 1993-1-1/NA.2.23). In other words, the deflection components w_g, inst from self-weight are subtracted. In this case, you can use the "Reduce number of load cases ..." option. In the load cases to be considered, all load cases are selected, except the self-weight. Confirm the subsequent query with [Yes] and you get the combinations without considering the self-weight.
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